- Currently, we create a request ID for internal use if one isn't set & this is used in logs
- If a custom request ID is set via X-Request-ID header, this gets logged, however, we don't return this with the response
- Means that a custom ID gets lost on the way back out, and makes tracing requests through a system trickier
- This change ensures that if X-Request-ID is set on the request, it is also set on the response so that requests can be properly traced
- It's easy to set this in e.g. nginx so that the feature becomes available - Ghost doens't need to do this
- Note: also split request id handling out into new middleware
